The present invention relates to a remote amplifier for a repeating type optical fiber transmission system and a method of evaluating the faulty point of an optical fiber.
Transmission systems using optical fibers are generally classified into two types, i.e., a repeating type system and a non-repeating type system. The repeating type system includes repeaters arranged at pre selected intervals on a transmission path connecting opposite end offices. The non-repeating type system connects opposite end offices without any repeater arranged on a transmission path. In a submarine optical fiber cable transmission system, for example, the repeating type system and non-repeating type system are respectively used for long distance, deep sea applications and short distance, shallow sea applications. While the non-repeating type system does not need repeaters or power supply devices therefore and saves cost, a transmission distance available with such a system is limited due to the absence of repeaters. A remote pumping system is a specific form of the non-repeating type system devised, to extend the transmissible distance. In the remote pumping system, a remote amplifier is inserted in a transmission path at a pre selected distance from opposite end offices. The remote amplifier receives exciting light, or pumping light, from either one of the end offices and amplifies signal light.
When a fault occurs in an optical fiber cable in the optical fiber transmission system, e.g., when the cable snaps or when a light loss increases, it is necessary that the opposite end stations be capable of evaluating the faulty point. In the non-repeating type system not effecting power supply, a faulty point is evaluated mainly by using optical fibers. A fault of an optical fiber cable is ascribable to the snapping of the cable itself or the cutting of one or more of optical fibers included in the cable. It is therefore desirable to evaluate a faulty point fiber by fiber.
Fiber-by-fiber faulty point evaluation can be done by using an ODOR (Optical Time Domain Reflect meter; optical pulse tester) which will be described specifically later. To evaluate a faulty point with an ODOR, optical test pulses are input to the end of a subject optical fiber at an end office. On reaching a faulty point of the fiber, the test pulses are reflected and diffused and returned to the end office. The resulting reflected light and backward diffused light are observed at the end office to determine the kind of the fault, e.g., cutting or an increase in light loss.
Japanese Patent Laid-Open Publication No. 9-261187 (document 1 hereinafter) teaches a specific remote amplifier and a specific method of evaluating a faulty point. In accordance with the document 1, it is possible to search for a faulty point in all the sections between end offices included in an optical communication system. However, the document 1 has the following problems unsolved. To evaluate a fault occurred at a certain point between remote amplifiers, the method evaluates the faulty point with only a single remote amplifier closer to the faulty point and therefore fails to evaluate the faulty point with accuracy. Moreover, the remote amplifier is constructed such that an optical signal for evaluating a faulty point is propagated through an erbium fiber. It follows that a propagation loss ascribable to the erbium fiber is great in a 1.3 xcexcm or a 1.55 xcexcm wavelength band particular to an ordinary ODOR. The method therefore needs a special 1.6 xcexcm wavelength band ODOR and increases the evaluation cost.
Summary of the Invention
It is therefore an object of the present invention to provide a method capable of enhancing accurate faulty point evaluation with an ordinary inexpensive ODOR.
A remote amplifier of the present invention includes a transmission port and an optical splitter connected to the transmission port via a transmission path. An optical combiner is connected to the optical splitter via an erbium fiber. A first branch port is connected to a branch path extending from the optical splitter.
With the above remote amplifier, it is possible to branch an optical signal being propagated through the transmission path and output or input it via the first branch port.
Further, a second branch port is connected to a branch path extending from the optical combiner. This also makes it possible to branch the optical signal being propagated through the transmission path and output or input it via the second branch port.
A faulty point evaluating method of the present invention begins with the step of sending signal light for faulty point evaluation from one end office to the above remote amplifier via an optical transmission path. The signal light is used to determine whether a fault has occurred in a first section between the receipt side of the remote amplifier and the end office or whether it has occurred in a second section between the transmission side of the remote amplifier and the other end office. Subsequently, signal light for faulty point evaluation is sent from one of two branch ports of the remote amplifier closer to the detected faulty section than the other. The resulting light reflected and diffused at the faulty point is returned to the above one branch port and detected for the evaluation of the fault. Because this method sends the light for evaluation without the intermediary of an erbium fiber, it is practicable with an ordinary ODOR and therefore reduces evaluation cost.